Apparatus which press against a workpiece that is to be sewn are generally known as presser-foot devices. These devices perform the function of preventing the material that is being sewn from lifting when the needle is being withdrawn from the workpiece. This tendency of the workpiece to lift with the needle will otherwise cause flagging and hence improper stitching.
Conventional presser-foot devices have for the most part been driven by various eccentric arrangements which move the presser-foot downwardly toward the workpiece in a timely fashion. This downward travel is usually limited to a pre-set height above the bed of the sewing machine. This pre-set height usually allows the presser-foot to move relatively close to a workpiece having a uniform thickness of one or more materials to be sewn. This close relationship of the presser-foot to the workpiece is maintained throughout the sewing period as long as the thickness does not vary appreciably.
Workpieces consisting of multiple layers of material usually present a problem for presser-feet having pre-set height limitations. A compromised setting must be used for sewing the different layers of these workpieces. Such a setting will result in the presser-foot making contact with the top layer of a two layer workpiece and substantially clearing the bottom layer. This compromised setting may or may not be sufficient to prevent flagging and hence improper stitching. The solution to this problem is often to lower the sewing speed which reduces the overall productivity of the machine. When the variable thickness of the workpiece exceeds two levels of thickness, a compromised setting is even less practical.
The problem encountered with sewing at various levels has been somewhat remedied by self-adjusting presser-foot devices. These devices are capable of adjusting to the different thicknesses of a workpiece so as to always maintain contact therewith. These devices vary as to their success in achieving a satisfactory pressurized contact. These devices are furthermore often complex and susceptible to extreme wear of their various parts. These complex mechanisms furthermore often dictate that the presser-foot must maintain a relationship relative to the needle which does not allow for an easy threading of the needle.